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Global biogeochemical cycles 19, — Google Scholar. Dividing the carbon cycle in these individual pieces help us represent this system in simple terms. The short term organic carbon cycle The photosynthesis reaction removes carbon atoms from the atmosphere and incorporates them into the living tissue of green plants. It requires energy derived from radiation in the visible part of the electromagnetic spectrum.
The chemical reaction, which students are responsible for learning, is on p. The terrestrial biosphere is much more massive than the marine biosphere, largely because of the presence of trees. Soils also contain a large amount of organic material. The influence of the land biosphere is evident in Fig. From October through January, when photosynthesis is largely confined to the tropics and the relatively small Southern Hemisphere continents, the respiration and decay reaction dominates and atmospheric carbon dioxide increases with time.
The marine biosphere operates like a 'biological pump'. In the sunlit uppermost meters of the ocean, photosynthesis serves as a source of oxygen and a sink for carbon dioxide and nutrients like phosphorous. Fecal pellets and dying marine organisms decay as they settle into the deeper layers of the ocean, consuming dissolved oxygen and giving off dissolved carbon dioxide.
Hence, these layers have much higher carbon dioxide concentrations and lower oxygen concentrations than the waters just below the surface as shown on p. The biological pump determines the carbon dioxide concentration of the water that is exposed to the atmosphere. The marine biosphere is active only in those limited regions of the ocean where upwelling is bringing up nutrients from below.
Once nutrients reach the sunlit upper layer of the ocean they are used up in a matter of days by explosive plankton blooms. The long term organic carbon cycle Only a tiny fraction of the organic material that is generated by photosynthesis each year escapes the decay process by being buried and ultimately incorporated into fossil fuel deposits or sediments containing more dilute fragments of organic material.
Through this slow process, carbon from both terrestrial and marine biosphere reservoirs enters into the long term organic carbon cycle. The rate is so slow as to be virtually unmeasurable. Weathering of these same sediments releases carbon back into the other reservoirs. Plants exchange carbon with the atmosphere relatively rapidly through photosynthesis, in which CO2 is absorbed and converted into new plant tissues, and respiration, where some fraction of the previously captured CO2 is released back to the atmosphere as a product of metabolism.
Of the various kinds of tissues produced by plants, woody stems such as those produced by trees have the greatest ability to store large amounts of carbon.
Wood is dense and trees can be large. Measuring soil carbon can be challenging, but a few basic assumptions can make estimating it much easier. First, the most prevalent form of carbon in the soil is organic carbon derived from dead plant materials and microorganisms. Second, as soil depth increases the abundance of organic carbon decreases. Standard soil measurements are typically only taken to 1m in depth. Most of the carbon in soils enters in the form of dead plant matter that is broken down by microorganisms during decay.
The decay process also released carbon back to the atmosphere because the metabolism of these microorganisms eventually breaks most of the organic matter all the way down to CO2. Fluxes are usually expressed as a rate with units of an amount of some substance being transferred over a certain period of time e. For example, the flow of water in a river can be thought of as a flux that transfers water from the land to the sea and can be measured in gallons per minute or cubic kilometers per year.
A single carbon pool can often have several fluxes both adding and removing carbon simultaneously. For example, the atmosphere has inflows from decomposition CO2 released by the breakdown of organic matter , forest fires and fossil fuel combustion and outflows from plant growth and uptake by the oceans.
The size of various fluxes can vary widely. In the previous section, we briefly discussed a few of the fluxes into and out of various global C pools. Here, we will pay more careful attention to some of the more important C fluxes. Photosynthesis: During photosynthesis, plants use energy from sunlight to combine CO2 from the atmosphere with water from the soil to create carbohydrates notice that the two parts of the word, carbo- and —hydrate, signify carbon and water.
In this way, CO2 is removed from the atmosphere and stored in the structure of plants. Virtually all of the organic matter on Earth was initially formed through this process. Because some plants can live to be tens, hundreds or sometimes even thousands of years old in the case of the longest-living trees , carbon may be stored, or sequestered, for relatively long periods of time.
When plants die, their tissues remain for a wide range of time periods. Tissues such as leaves, which have a high quality for decomposer organisms, tend to decay quickly, while more resistant structures, such as wood can persist much longer. Plant Respiration: Plants also release CO2 back to the atmosphere through the process of respiration the plant equivalent of exhaling. Respiration occurs as plant cells use carbohydrates, made during photosynthesis, for energy.
Litterfall: In addition to the death of whole plants, living plants also shed some portion of their leaves, roots and branches each year. Because all parts of the plant are made up of carbon, the loss of these parts to the ground is a transfer of carbon a flux from the plant to the soil.
Dead plant material is often referred to as litter leaf litter, branch litter, etc. Soil Respiration: The release of CO2 through respiration is not unique to plants, but is something all organisms do.
Because it can take years for a plant to decompose or decades in the case of large trees , carbon is temporarily stored in the organic matter of soil. It may not seem obvious that gasses can be dissolved into, or released from water, but this is what leads to the formation of bubbles that appear in a glass of water left to sit for a long enough period of time.
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